Optical fiber continues to evolve as a transport mechanism in a variety of applications including, but not limited to, communication systems, industrial and environmental sensors, medical imaging and the like. More applications continue to be uncovered daily. When first used in telecommunication applications, the characteristics of a basic silica fiber (in terms of core/cladding dimensions, refractive index profile, etc.) were relatively simple to be controlled using processes such as modified chemical vapor deposition (MCVD) and vapor-assisted deposition (VAD) developed at Bell Laboratories in the 1970's and 80's (see, for example, U.S. Pat. Nos. 4,257,797; 4,707,174; and 4,932,990). The growth in variety of applications and types of fiber has resulted in the need to develop specialized fabrication processes. Besides conventional single mode fiber (SMF) and multimode fiber (MMF), fibers such as dispersion-shifted fiber (DSF), dispersion-compensating fiber (DCF), polarization-maintaining fiber (PMF), microstructured fiber, large-mode-area fiber (LMA fiber), higher-order-mode fiber (HOM fiber), doped fiber amplifiers and highly-nonlinear fiber (HNLF) have been developed to meet particular needs. Fibers in one or more of these types have also been formed to include multiple, separated core regions.
In each case, the different fiber types have developed a particular set of characteristics, in terms of the dimensions of the various core and cladding layers, materials and dopants used to form these layers and processes used to create the specialized layers. While various specialized processes have been developed to fabricate specialized preforms from which these fibers are drawn, the as-drawn fiber may exhibit length-dependent variations which are unacceptable. Length-dependent variations in nonlinear characteristics such as chromatic dispersion, polarization mode dispersion, birefringence, zero dispersion wavelength, cutoff wavelength (for both fundamental and higher-order modes), stimulated Brillouin scattering (SBS) characteristics and the like have limited the capabilities of these fibers. In some applications where only short lengths of fiber are used, the length-dependent variations are of little concern. However, in applications where relatively long lengths of fiber are required, any significant length-dependent variation in one or more of these characteristics may be problematic.
For example, HNLF is a dispersion-shifted fiber with a small effective area (Aeff) for nonlinear applications such as, for example, continuum generation and four-wave mixing (FWM). U.S. Pat. Nos. 6,775,447 and 7,171,089—assigned to the assignee of this application—describe the utilization of HNLF for these activities. As discussed in these and other prior art references, relatively short lengths of HNLF are used to perform continuum generation and FWM. However, when it is desired to use longer lengths of HNLF for other applications, the non-uniformity of the chromatic dispersion profile as a function of fiber length becomes problematic. At least one source of the variation in dispersion can be attributed to changes in the fiber diameter during fabrication (e.g., drawing from a fiber preform into the final fiber), where slight changes in diameter have been found to introduce significant changes in the dispersion.
A sensitivity analysis on the design of HNLF shows that if the zero-dispersion wavelength (ZDW) of the fiber is required to change no more than 0.1 nm over a 1 km length, the fiber diameter must not change by more than 0.002%. For a 125 μm fiber, this translates to a diameter variation of less than 2.8 nm over 1 km.
One prior art method for modifying the diameter of a drawn fiber is disclosed in U.S. Pat. No. 7,317,856 issued to M. Hirano et al. on Jan. 8, 2008. While not directed to the fabrication of HNLF or its peculiar requirements, the Hirano et al. method does provide an analysis of a fiber preform and thereafter modify its diameter to provide “desired” optical characteristics. In particular, the refractive index profile of the fiber preform is analyzed and demarcations made on the preform to indicate regions where the profile is not uniform. Thereafter, the preform outer surface is subjected to a grinding operation to re-create a uniform profile and the ground preform is drawn down into a fiber with an essentially uniform refractive index profile.
While this method is suitable for use in improving the uniformity of the refractive index profile of a standard profile, such a method is not considered useful in controlling various parameters in many of today's specialized fibers, where the draw process itself is known to introduce slight variations in fiber diameter sufficient to create undesired length-dependent variations.
Additionally, there are situations where it is desired to introduce a post-drawn modification to the fiber characteristics. For example, in a multi-core fiber it may be desired to adjust the coupling between the cores in a case-by-case (i.e., application-specific) basis, or adjust the inter-core coupling as a function of length. In some instances, a non-constant but controlled refractive index profile is desirable, such as providing an axial variation in index which anticipates the axially-varying optical field propagating through the fiber. Moreover, it would be desirable to modify certain characteristics of an optical fiber at the time of system installation or field deployment (for example) such that the optical fiber's characteristics are particularly tailored to the needs of the specific application and related conditions.
Thus, a need remains in the art for a method of providing modifications to an optical fiber's characteristics to particularly tailor the refractive index profile of an optical fiber for a specific fiber type and application.